1. Field of the Invention
The present invention relates to a radio network controller for controlling a plurality of base stations communicating with a mobile terminal via radio in a mobile communication network. The invention also relates to a method for inter-area handover that enables a mobile terminal to continue ongoing communication without interruption when the mobile terminal moves between areas served by different base stations.
2. Description of the Related Art
In a mobile communication network capable of accommodating third generation mobile telephones such as W-CDMA, the technical specifications for which are defined by 3GPP (3rd Generation Partnership Projects), when a mobile terminal moves between areas served by different base stations, the mobile terminal is temporarily connected to both base stations. FIG. 1 is a diagram schematically showing the configuration of a mobile communication network capable of accommodating third generation mobile telephones.
As shown in FIG. 1, the mobile communication network 1 includes: a plurality of base stations BS1, BS2 which communicate with a mobile terminal MT via radio; a serving radio network controller (SRNC) 100 which controls the base stations; a fixed communication network N; and an exchange 200 connecting between the fixed communication network N and the radio network controller 100.
In the condition shown here, the mobile terminal MT is moving from an area A1 served by the first base station BS1 to an area A2 served by the second base station BS2; at this time, the mobile terminal MT is temporarily connected to both base stations.
As a result, the same data transmitted from the mobile terminal MT arrives at the radio network controller 100 via two different paths, i.e., the path 1 originating from the first base station BS1 and the path 2 originating from the second base station BS2.
The radio network controller 100 compares the data qualities of the same data received via the different paths, and selects the higher quality data for use. This improves the communication quality at the time of area switching occurring in connection with the movement of the mobile terminal MT.
In the above case, the data transmitted via the different paths 1 and 2 arrive at the radio network controller 100 at somewhat different times due to the difference between the physical distances that the data travel along the respective paths. The radio network controller 100 performs buffering to accommodate this difference. That is, the radio network controller 100 causes the data received from the shorter data transmission path to wait until the data from the longer data transmission path arrives. How this is done will be described with reference to FIG. 2.
The data to be transmitted from the mobile terminal MT is divided into data transmission units called frames D1 to D6; the frames D1 to D6 transmitted via the path 1 are received by the radio network controller 100 at times t1 to t6, respectively. On the other hand, the frames D1 to D6 transmitted via the path 2 are received by the radio network controller 100 at times t7 to t12, respectively, with a time delay ΔT of nearly four frame periods with respect to the times t1 to t6 at which the same frames transmitted via the path 1 were received. Therefore, buffers B1 to B4 for buffering four frames are provided in the radio network controller 100.
In this patent specification and the appended claims, the term “frame” is used to refer to data whose amount corresponds to one data transmission unit in the data transmission between the mobile terminal MT and the radio network controller 100. It is therefore to be understood that the term, when written simply as “frame”, may be used to refer to only the payload which is the main body of the transmitted data, as well as to the data containing the payload with header information appended thereto.
When the frame D1 is received at time t1, the radio network controller 100 stores the frame D1 in the buffer B1. Likewise, when the frames D2 to D4 are received at times t2 to t4, respectively, the received frames D2 to D4 are stored in the respective buffers B2 to B4.
Thereafter, when the frame D1 transmitted via the path 2 is received at time t7, the radio network controller 100 reads the buffer B1 to retrieve the frame D1 received via the path 1, compares the data quality between these frames, and selects the higher quality frame for transmission (time t13). At this time, the buffer B1 is emptied, so that the next frame D5 arriving via the path 1 is stored in the buffer B1. Thereafter, the same operation is repeated.